In an era of coed dorms and slackening rules about "overnight guests," a new constituency has emerged on college campuses: the roommate inconvenienced by sex.

Shielding a cohabitant from moments of intimacy is a part of collegiate etiquette as old as the sexual revolution. In the previous generation, a tie or sock hung from a doorknob served as a do-not-disturb sign. Today, the warning is more commonly delivered by text message. At some point, students displaced from their rooms came to be known as "sexiles."

Schools across Washington have mostly tiptoed around the issue of roommate sex, reminding students in general terms of the need for common civility. No student at St. John's College in Annapolis may "knowingly interfere with the sleep or study" of another. Students at Washington and Lee University in Lexington, Va., are told to mind the "rights and sensibilities of others." Students at the College of William and Mary in Williamsburg sign a roommate agreement that covers overnight guests along with cleanliness and borrowed food.

Tufts University, near Boston, raised eyebrows this fall by going where its peers would not. The school has banned sexual activity in dorm rooms when a roommate is present.

"It happens, not often, but it happens enough," said Ben Gittleson of Gaithersburg, a 20-year-old Tufts junior. "I think it's ridiculous that people can't talk it out with their roommates. But people aren't always considerate, and roommates aren't always assertive enough."

Tufts might be the first college in the nation to make explicit what other schools have only hinted at: It is not cool to have sex in front of your roommate.

The action also is notable as a baby-step backward in the decades-long march toward fully coeducational living. Colleges have steadily loosened rules about romantic guests as they have brought the sexes closer together, first in coed dorms, then coed floors and finally coed rooms. Couples can now share a dorm room at a growing number of schools, including Wesleyan University and Oberlin College. (Oberlin also hosts an annual Safer Sex Night.) Students can share an apartment at American University and at the University of Maryland. School officials note that the trend toward mixed-sex housing is intended to promote neutrality, not romantic cohabitation.

The policy at Tufts, a private research university with 9,500 students, has reaped heavy publicity on college campuses, providing fodder for Conan and Leno and reducing the school to something of a national punch line. Tufts is not regarded as a particularly socially liberal school, apart from an eccentric annual winter ritual known as the Naked Quad Run.

Tufts officials said the change was prompted by persistent complaints from students, numbering perhaps a dozen over the past two to three years.

In response, administrators helpfully added a new item to a list of host responsibilities for students with overnight guests: "You may not engage in sexual activity while your roommate is present in the room." Dorm sex should never "deprive your roommate(s) of privacy, study, or sleep time."

Tufts officials hope the new policy will "empower roommates to initiate conversations about what may be uncomfortable subjects," said Kim Thurler, a spokeswoman. "When students share a room, they must have good, clear communication about how the room will be utilized."

Tufts officials said they know of no other school that has recognized a student's right to be spared roommate sex. It's too early to tell whether their new rules might set a trend.

The policy has not been universally embraced. Some at Tufts deem the topic a bit too personal and think it should be left to roommates to sort out. Others resent that the schoolwide community was not involved in the decision. Still others -- those too shy to speak up or with particularly thoughtless cohabitants -- feel as though a burden has been lifted.

Martha Shanahan, a Tufts freshman from the District, recalls the plight of a classmate who was driven from her room by a sexually active roommate.

"Just within a few weeks of school starting, she was sleeping on a couch in our room because her roommate was in there with a boy," said Shanahan, 18.

The classmate, who withheld her name to protect delicate roommate relationships, said her cohabitant appeared with a male companion one night about 3 a.m., seeking privacy, "and I didn't know her well enough at that point that I could confront her."

The two later came to terms and signed a roommate contract provided by the resident adviser. "We've gotten along great since. We've moved past it," she said.

Among local colleges, Georgetown University has come closest to positing a bill of rights for sexiles. The school advises students that "cohabitation, which is defined as overnight visits with a sexual partner, is incompatible both with the Catholic character of the University and with the rights of the roommates."

Other colleges generally head off conflict with rules that require a roommate's permission for overnight guests. A few institutions explicitly forbid sex in the dorms, including Catholic University of America and the U.S. Naval Academy. The University of Maryland advises roommates to "learn to negotiate and open the lines of communication," not just about sex but also hours of study and sleep, television and computer use, and the sharing of clothing and iPods.

"You're sharing that space. It's a tight, small space. You have to do some compromising," said Deb Boykin, assistant vice president for student affairs at William and Mary.

Conflicts over roommate sex arise perhaps once a year at William and Mary, Boykin said, and are easily outnumbered by complaints about cleanliness and purloined food. Students are more likely to find a new roommate, without explanation, than to try to fix an incompatible pairing.

Students tend to follow "kind of an unspoken code" in coordinating with roommates to allow opportunities for intimacy with minimal inconvenience, said Marshal Staggs, 20, a senior at George Washington University and occasional sex columnist for the Hatchet student newspaper. A student seeking privacy will dispatch a discreet text message, and a cooperative roommate will go willingly into sexile.

"It's not really the sock-on-the-door thing anymore," Staggs said. "It's a little more direct than that now."

Even so, Staggs has heard from students who have borne reluctant witness to intimacy.

"Sometimes, people will just pretend that they're still asleep," he said. "They'll put on their iPod or something like that."